Positron emission tomography using 2-[18F]-deoxyglucose was used to measure regional functional brain activities in adult patients with Generalized Thyroid Resistance Disorder (RTH) during the performance of an auditory discrimination task (CPT). RTH patients were found to perform poorly on the CPT and to have an abnormally elevated cingulate cortex activity. A change from elevated to reduced cingulate activity in the performance of normals is expected on the basis of a shift between Posner's hypothesized anterior and posterior attention networks. Since the anterior attention network functions at birth, but the posterior network appears to develop after birth, the results are consistent with the previously established importance of thyroid hormone in brain development during the post-natal period. Positron emission tomography was used to measure presynaptic accumulation of the tracer 6-[18F]-DOPA in the dopaminergic regions of the brains of 12 patients with Lesch-Nyhan disease and 15 healthy controls. Lesch- Nyhan children were found to have significantly lower presynaptic dopaminergic activity in the basal ganglia, frontal cortex and substantia nigra. There was no overlap in the LN and normal control groups. The degree of deficit found was as great as that observed in Parkinson's Disease, and not restricted to the basal ganglia. This suggests that dopaminergic deficits make important contributions to the characteristic neuropsychiatric syndrome of LN. The fact that the deficits were already present in the youngest of the patients scanned (10 y/o) with no apparent change with age suggests that they are neurodevelopmental in origin as opposed to degenerative in nature.